1. Field of the Invention
This invention relates to a spark plug and a method of making the spark plug in which a spark-erosion resistant noble metal is secured to a front end of a center electrode.
2. Description of Prior Art
In a spark plug for an internal combustion engine, a noble metal tip is welded to a front end of a center electrode or a discharge end of a ground electrode so as to increase a spark-erosion resistant property. The noble metal tip is usually made of Pt, Pd, Ir or alloys with these metals as main components. The tip is bonded to the electrode by means of electric resistance welding, and then the tip is extruded together with the front end of the center electrode as shown in Japanese Patent Application Publication No. 62-31797.
The electric resistance welding forms a diffused alloy layer in which the noble metal is diffused into the front end of the center electrode. Considering that the center electrode is made of a nickel-based alloy, a stress concentration occurs at an interface between the noble metal tip and the front end of the center electrode due to a repetitive thermal stress caused by thermal expansional difference therebetween when the center electrode is repeatedly exposed to the heat-cool cycle in a combustion chamber.
For this reason, there arises a possibility of developing cracks at the interface between the tip and the center electrode to break the tip off the center electrode when the cracks sufficiently grow.
Therefore, it is one of the objects of the invention to provide a spark plug and a method of making the spark plug in which a noble metal portion is positively secured to an outer side wall of a front end of a center electrode by means of laser beams so as to prevent the noble metal portion from inadvertently peeling off the center electrode, and thus contributing to an extended service life with relatively low cost.